Traditionally, xylitol has been produced by the crystallization of a saturated aequeous solution of xylitol to form single crystals, tetrahedron in shape, of relatively uniform size. These crystals are grown in solution, separated from solution by centrifuging, drying the crystals and then grinding them into a powder. Thus, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Third Ed., (1978), John Wiley & Sons, indicate (at Vol. I, page 766) that "Xylose is obtained from sulfite liquors, particularly from hardwoods, such as birch, by methanol extraction of concentrates or dried sulfite lyes, ultrafiltration and reverse osmosis, ion exchange, ion exclusion, or combinations of these treatments . . . Hydrogenation of xylose to xylitol is carried out in aqueous solution, usually at basic pH, with Raney nickel catalyst at a loading of 2%, at 125.degree. C. and 3.5 MPa (515 psi)," An alternative aqueous crystallization process is described in U.S. Pat. No. 3,985,515 to Jaffe et al.
Xylitol produced in this fashion is typically moisture sensitive and is thus prone to caking, making it difficult to use such product after periods of storage due to the clumping that typically occurs. When xylitol powder is employed in sucrose free chewing gum, where it is used as part of the sweetner/bulking agent as a replacement for sugar, manufacturers may typically bring in xylitol crystallized from solution in bulk, unground form then grind it down themselves, just prior to addition to the gum batch, in order to minimize caking and the problems associated therewith.
Moreover, due to its crystalline structure, i.e., distinct single crystal, definitive form, and very dense nature, when added to gum, aqueous crystallized xylitol does not "dry" the gum out and even with reductions in plasticizer, the gum is typically very soft and difficult to handle/process in gum plants and the gum produced has a coarse texture. At the typical use levels of 7-15% (based on total weight), gum containing aqueous crystallized xylitol poses unique handling problems.
Another characteristic of the single crystalline form of aqueous crystallized xylitol is that such structure does not allow for any "copenetration" of the crystals to effect a bond of the crystals during the preparation of a dried compression tablet. The dense nature of the crystal results in crystals with very low plastic deformation characteristics or values and the bonding energy of the crystal is low. As a result, it is not possible to make a direct compression tablet having an acceptable degree of hardness and texture from xylitol powder produced from an aqueous crystallization process. Rather, one must first wet the xylitol powder with water in a high velocity air stream to form an agglomerate, and then dry and size the resulting product. Product produced by this process can be used in direct compression applications to make tablets of good hardness and durability. It is an added expense however to have to agglomerate the product from the ground aequeous crystallized xylitol. The added expense plus the "grittiness" that is acquired in the agglomeration process has greatly limited the use of xylitol as an excipient for tablet manufacture.
Accordingly, it would be greatly desirable to produce a novel form of crystalline xylitol which, when ground to a powder, exhibited increased moisture resistance and correspondingly less of a tendency to cake. It would further be greatly desirable to process a novel form of crystalline xylitol which had a unique crystalline habit or a less dense crystalline structure such that it could easily be formulated into or directly compressed into tablets without requiring an intervening agglomeration and grinding step, or formulated into chewing gum having reduced grittiness and tackiness.
Accordingly, it is an object of this invention to provide a novel crystalline form of xylitol having increased resistance to moisture pickup.
It is a further object of this invention to provide a novel form of crystalline xylitol which can be more easily formulated into chewing gum.
It is yet another object of this invention to provide a crystalline xylitol which can be directly compressed into tablets having an acceptable degree of hardness without the need for an intermediate agglomeration step.
It is yet a further object of this invention to provide a method of producing such a novel form of xylitol.
These objects and other additional objects will become more fully apparent from the following description and accompanying Examples.